Aging Affects Muscarinic Agonist-induced Fluid Secretion From Submandibular Gland

Objectives: The effect of aging on salivary gland function is not well-described. This study aimed to clarify how aging affects salivary gland function using the senescence-accelerated mouse prone1 (SAMP1).
Methods: SAMP1, aged 16 (control) and 48 (aged group) weeks old, were examined. To evaluate parotid gland (PG) and submandibular gland (SMG) function, we performed an in vivo analysis with intraperitoneal muscarinic agonist pilocarpine stimulation. Ex vivo SMG perfusion with the muscarinic agonist carbachol or the β-adrenergic agonist isoproterenol were used to evaluate detailed SMG function. An increase in [Ca²⁺]_i was observed in Fura2-AM-loaded SMG acinar cells. Histological analyses were performed by hematoxylin-eosin staining and immunohistochemical staining for aquaporin-5 (AQP5), Na⁺-K⁺-2Cl^- cotransporter 1 (NKCC1), and transmembrane protein 16A (TMEM16A). Statistical significance was determined by Student’s t-test. Experiments were approved by the Animal Committee of Kyushu Dental University (No. 15-021).
Results: Pilocarpine-induced saliva from in vivo PG was comparable between the 16- and 48-week-old mice, whereas saliva from in vivo SMG was significantly less in 48-week-old mice than in 16-week-old mice (p=0.005). Carbachol-induced saliva from ex vivo SMG was significantly less in 48-week-old mice than in 16-week-old mice (p=0.036). In contrast, simultaneous stimulation with carbachol and isoproterenol induced comparable saliva from the SMG of both groups. The carbachol-induced [Ca²⁺]_i increase in 48-week-old mice was comparable to that in 16-week-old mice. We observed lymphocyte infiltration in the SMG from 48-week-old mice, suggesting that chronic inflammation occurs in the aged SMG. Immunohistochemistry revealed that AQP5, NKCC1, and TMEM16A were expressed in the SMG from both groups and there was no difference between the two groups.
Conclusions: These results suggest that aging affects SMG but not PG function. Muscarinic stimulation-induced saliva from the SMG was diminished by aging; however, aging did not decrease salivary secretion induced by simultaneous stimulation with muscarinic and β-adrenergic agonist.